1. Field of the Invention
This invention relates to an apparatus and process for effecting control of mercury emissions from exhaust gases resulting from the operation of coal-fired and waste combustors. These mercury emissions include elemental mercury [Hg(0)] and oxidized mercury species, usually mercury(II)chloride. Enhanced control of mercury emissions from combustors is directly linked to the proportion of oxidized mercury, i.e. mercury(II)chloride, present relative to elemental mercury in the exhaust gas. The mercury(II)chloride is sorbed more readily than is elemental mercury and thus can be more readily trapped from a flue gas stream or the like by adsorption onto a solid mass or by dissolution in wet processes. Therefore the mercury(II)chloride is subject to removal by conventional pollution control technologies which are currently being used by utilities, incinerators and similar facilities. Elemental mercury, however, is able to evade these control technologies.
In particular, the method of the present invention involves the previously unrecognized property of the noble metal, gold, to amalgamate Hg(0) and upon saturation and in the presence of dilute HCl or Cl.sub.2 gas to catalytically oxidize dilute concentrations of elemental mercury [Hg(0)] to mercury(II)chloride in a combustion exhaust gas. This is significant in that, post combustion, the dilute concentrations of Hg(0) and HCl detected in an exhaust gas do not interact measurably in the presence of water vapor at the concentrations typically found in an exhaust gas. However, the aforementioned exhaust gas matrix, when contacted with a noble metal coated substrate preferably a gold-coated substrate, will generate mercury(II)chloride, which thereafter can be removed by conventional pollution control technologies. By this method substantially complete mercury removal can be achieved.
2. Description of the Prior Art
The form of mercury which results as a consequence of a combustion process depends principally on both the temperature and composition of the combusted material. The results of thermochemical calculations show that at the temperatures which are typically found in most combustors, elemental mercury is formed. Nonetheless, such elemental mercury may be, and oftentimes is, converted to other forms of mercury as such elemental mercury is transported from the combustor to the exhaust stack.
The composition of the stack gases determines the distribution between Hg(0) and HgCl.sub.2. For example, the primary pollutant in incinerator flue gas is often HCl and combustion systems containing large amounts of chloride in the fuel will convert Hg(0) to oxidized forms of mercury, in particular, mercury(II)chloride [HgCl.sub.2 ]. The dominating pollutant from coal combustion is, however, generally SO.sub.2. These and other exhaust gas components affect the distribution of mercury species such that mercury in a municipal incinerator exhaust gas exists primarily as mercury(II)chloride, whereas the mercury content of a coal-fired combustor is primarily elemental mercury.
The effectiveness of mercury control operations depends on the proportion of mercury that is present in the form of mercury(II)chloride. Mercury(II)chloride is more soluble in aqueous solution than elemental mercury and can be removed by wet scrubbing systems. Mercury(II)chloride is also less volatile than Hg(0) and will sorb more readily onto particulates, such as activated carbon.
One approach to achieving an increase in the total removal of mercury, i.e. Hg(0) and HgCl.sub.2, from coal-fired and waste combustors is to develop a more effective means to remove Hg(0) since it is the more difficult to remove. One method is to contact an exhaust stream with sorbents that have been chemically modified to promote a reaction with elemental mercury. Revoir et al., U.S. Pat. No. 3,662,523, May 16, 1972, teach one method for improving the removal of Hg(0) from flue gas, by allowing the flue gas matrix to flow through a fixed sorbent bed composed of activated carbon that has been impregnated with halogen or inter-halogen compounds. Similarly, Dreibelbis et al., in U.S. Pat. No. 3,194,629, July 1965, teach activated carbon impregnated with potassium triiodine or sulfur removes Hg(0); or Kasai et al., in U.S. Pat. No. 3,876,393, Apr. 8, 1975, teach the use of activated carbon with impregnated sulfuric acid; or Manes et al., in U.S. Pat. No. 3,193,987, July 1965, teach the use of activated carbon impregnated with gold, silver or silver salts, copper or copper salts; or Matviya et al., in U.S. Pat. No. 4,708,853, Nov. 24, 1987, teach the use of molecular sieves impregnated with sulfur; or Ambrosini et al., in U.S. Pat. No. 4,101,631, Jul. 18, 1978, teach the use of zeolites impregnated with elemental sulfur. Alternatively, sorbents to remove Hg(0) may be injected into the flue stream. The impact of an injection of chemically modified sorbents to trap Hg(0) has not been evaluated for electrostatic precipitator (ESP) and baghouse equipment in coal-fired utilities, but sorbent injection has been found to decrease the efficiency of baghouse and ESP equipment in incinerator processes. Additionally, the cost of these impregnated sorbents can be five times that of activated charcoal.
Another approach to increase total mercury removal is to oxidize Hg(0), producing a species that is easier to capture by other emission control directives. It is well-known that Cl.sub.2 or HCl readily oxidizes Hg(0), even at room temperature. However, the simple addition of Cl.sub.2 or HCl to a flue gas does not lead to the oxidation of Hg(0) to produce HgCl.sub.2. The amount of water vapor present in the gaseous matrix absorbs the gaseous Cl.sub.2 or HCl and thereby hinders the interaction with Hg(0). Ide et al., U.S. Pat. No. 4,729,882, Mar. 8, 1988, teach a chloride-containing material to be effective in the oxidation of Hg(0) provided the said material is fed into the combustion zone. A process is described in which a chlorine-containing material, such as Cl.sub.2, HCl, plastics or salts, is added to a vessel holding the mercury-containing material and the mixture is heated to convert the mercury into mercuric chloride which is removed by scrubbing with wash water. Felsvang et al., U.S. Pat. No. 5,435,980, Jul. 25, 1995, teach that by increasing the chloride concentration to the drying-absorption zone of a spray dryer absorber to a quantity sufficient to oxidize Hg(0) to HgCl.sub.2, mercury removal from the flue gas is enhanced. A disadvantage to these processes is that the quantity of HCl or Cl.sub.2 which must be added in order to effect the desired degree of oxidation of the Hg(0) in these processes could increase the corrosion downstream therein of the stack associated equipment.
Yet another approach is disclosed by Durham et al., in U.S. Pat. No. 5,409,522, Apr. 25, 1995, which teach that all mercury species in a flue gas can be captured by sorption onto a preferred collection surface and wherein such surface preferably comprises gold. The resulting accumulated mercury can be quantitatively desorbed by heating the gold to temperatures of 700.degree. C. and above. This property is applied in some commercially-available analytical instruments in which the mercury is measured in ambient air. However very little chemistry has been evaluated between mercury and gold under the extremely corrosive conditions found in a flue gas stream. These hostile conditions can result from such factors as the varying concentrations of acid forming flue gas components including hydrogen chloride (HCl) and sulfur dioxide (SO.sub.2), which are dependent on combustor loading and the type or source of coal, coupled with the high temperatures usually found in flue gas stacks. For instance, a combination of high moisture levels, i.e. ranging from about 7% to about 10% H.sub.2 O, in the presence of HCl results in a flue gas which is known to be particularly corrosive. According to Durham et al., '522, supra, the mercury species collected by the gold surface is subsequently regenerated by isolating the surface from the flue gas stream, elevating the temperature of the gold surface from 500.degree. F. to 800.degree. F. (260.degree. C. to 427.degree. C.), preferably from 650.degree. F. to 800.degree. F. (343.degree. C. to 427.degree. C.), to release sorbed mercury in the form of Hg(0) into an input gas stream and subsequently condensing the elemental mercury vapor at a temperature below its dew point. However, previous investigations on the collection of mercury(II)chloride by gold surfaces (F. Slemr, W. Seiler, C. Eberling and P. Roggendorf, "The Determination of Total Gaseous Mercury in Air at Background Levels," Anal. Chim. Acta, Vol. 110, p. 35, 1979) indicated that only about 50% of HgCl.sub.2 was released at 700.degree. C. as Hg(0). Consequently the presence of mercury(II)chloride in the flue gas can further complicate such use of gold due to the uncertainty of its interaction therewith. It is also known that chlorine reacts with gold at temperatures up to about 300.degree. C. to form gold monochloride. Indeed, this phenomenon is utilized in the recovery of gold from gold alloys, U.S. Pat. No. 5,004,500, Dunn et al., Apr. 2, 1991. Similar gold reactions occur in the presence of HCl gas.
U.S. Pat. No. 5,750,992, Van Pelt el. al., May 12, 1998, assigned to the assignee of the instant invention, discloses a method for the measurement of elemental mercury in a flue gas. There are components in the flue gas that interfere with Hg(0) measurement. To eliminate these, the flue gas (which has been dewatered to remove most of the HCl and HgCl.sub.2 present) is passed through a gold cartridge which removes only Hg(0) and creates a reference gas that makes possible the measurement of Hg(0) in flue gas. We noted the time required to saturate a gold-coated sand with Hg(0) from a simulated flue gas which contained HCl gas was apparently infinite. By reducing the HCl gas concentration to 0 ppm in the simulated flue gas stream, an immediate increase of the Hg(0) concentration in the simulated flue gas stream was monitored by a UV photometer. Upon re-addition of the HCl gas to the simulated flue gas stream the Hg(0) absorption disappeared. We determined thereafter that during the saturation of the gold by Hg(0), an interaction of Hg(0) with the HCl occurred in the simulated flue gas that resulted in the oxidation of Hg(0) to HgCl.sub.2. The HgCl.sub.2 was desorbed into the gas matrix and undetected by the UV photometer. These data suggest that Durham, '522, supra, should present mercury collection problems under conditions that may be used in their patent.
In view of the consideration of the problems enumerated, supra, it should now be readily appreciated by those skilled in this art that there still exists a definite need for the development of a new, cost-effective and efficient method which will allow for the removal of mercury species, which are contained in flue stack gas streams normally emitted from waste and fossil-fueled combustors, including power plants, so as to reduce or substantially reduce the emissions to the environment of mercury from such combustors.